Ronny Paulino finished with three hits, and David Wright and Jason Bay each had two as the Mets scored four times in the first inning en route to their fifth straight win. It was New York's major-league leading 33rd road win.
The Nationals have lost six straight and eight of nine overall.
Dillon Gee (10-3) allowed three runs and four hits over 6 2/3 innings and hit an RBI single in the fourth.
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Wang (0-1) couldn't help the Nationals in his first major league appearance two years to the day after he had shoulder surgery. The right-hander gave up six runs on eight hits in four innings.
Jason Isringhausen pitched the ninth for his fifth save.
Wang, made his first start since July 4, 2009, when he was a member of the New York Yankees.
His rocky first inning started with a four-pitch walk to the leadoff hitter Reyes. The Mets shortstop took second on a wild pitch, moved to third on Justin Turner's single to center and scored on Daniel Murphy opposite field hit to left. It was the fourth straight game they pushed across a run in the first inning.
David Wright hammered a 1-2 slider off the right field wall for a single to load the bases. Angel Pagan hit a two-run single to left on the next pitch for the Mets' fourth consecutive hit.
Wang finally recorded an out on Jason Bay's grounder, but Lucas Duda's sacrifice fly to left scored Wright to make it 4-0. Pagan was caught between second and third and tagged to end the inning.
The Mets put up two runs in the fourth, Wang's final inning. With runners on first and second and no out, Paulino singled to right. Jayson Werth misplayed the ball, but centerfielder Rick Ankiel backed him up and threw Bay out at the plate. The Mets still scored two runs on Gee's grounder up the middle and Reyes' sacrifice to left.
Wang threw 60 pitches with one walk and two strikeouts.
Paulino and Reyes hit RBI doubles in the sixth inning off reliever Tom Gorzelanny.
After Gee, who walked four and struck out five, faced one above the minimum batters through three innings, the Nationals got on the board in the fourth. With the bases loaded and one out, Laynce Nix grounded to second base for a potential inning-ending double play, but Reyes' throw to first base for the final out was wild, allowing two runs to score.
In the fourth, Ankiel hit a sacrifice fly.
The Nationals plated two runs off three Mets relievers in the eighth to make it 8-5 and had the bases loaded with one out. D.J. Carrasco entered and struck out Danny Espinosa and Ryan Zimmerman to end the threat.
Wright has gotten a hit in all eight games since coming of the disabled list on July 22, and has multiple hits in seven of the eight games. ... Nationals OF Roger Bernadina was optioned to Triple-A Syracuse to make room for Wang. The Nationals' primary CF since Davey Johnson was hired as manager, Bernandina was hitting .247 with five homers, 20 RBI and 16 stolen bases in 71 games this season, but was hitless over his last 20 at-bats.