Michael Cammalleri, P.K. Subban and Lars Eller also scored for Montreal (4-3-0) in the final preseason game for both teams. Cammalleri, who also had an assist, was ejected at 16:52 of the second period for a hard slash to the ankle of Islanders prospect Nino Niederreiter. The 18-year-old Swiss forward limped off the ice and did not return.
Plekanec, who also had an assist, led the Canadiens with six goals and two assists in four preseason games.
Jon Sim and Jesse Joensuu scored for New York (1-4-0), which sent its 'B' team here without top players like John Tavares, Matt Moulson and Josh Bailey. They played in the club's other game, a 2-1 win at home against New Jersey.
It was technically a home game for the Islanders, but the Canadiens wore their red home jerseys and had the crowd on their side for a mostly one-sided game.
Carey Price was sharp on the few dangerous chances by the Islanders.
New York had some of its best prospects on hand, including defensemen Travis Hamonic and Niederreiter, the fifth overall pick in the June draft who worked a give-and-go with Sim on New York's first goal at 10:55 of the first period.
Cammalleri gave the Canadiens a 1-0 lead at 3:25 and a turnover let Jeff Halpern set up Pouliot for a goal 1:30 later.
After Sim's goal, Subban and Plekanec each beat Manny Legace on power plays - a sign of progress for a team that was 3 for 37 coming in.
Plekanec scored again 2:42 into the third on the first shot against Mikko Koskinen, Pouliot added another on a power play at 13:28 and rookie Eller got his first with the Canadiens as he beat Koskinen with a wrist shot at 16:48.
Joensuu beat Price from the slot with 1:20 left to play.
It was a big day for hockey as thousands marched in Quebec City in support of a new arena they hope will bring back the defunct Nordiques. Several former Nords were on hand, including brothers Peter, Marian and Anton Stastny, as well as Michel Goulet. ... The Canadiens left after the game for Malbaie, Quebec, for four days of bonding before the regular season. ... The Canadiens had a nearly full lineup, missing only Brian Gionta and Scott Gomez.