The Habs broke a 1-1 tie at the 6:49 mark of the final frame when Jarred Tinordi's shot from the high slot was redirected into the net by Plekanec.
Colby Armstrong got the other goal for Eastern Conference-leading Montreal, which equaled its longest winning streak of the season.
Carey Price turned aside 32 shots in the win.
Peter Harrold scored and Johan Hedberg made 20 saves for the Devils, who dropped a second consecutive game.
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