Roloson stopped all 16 shots he faced in the second period and 14-of-15 in the final frame for the Islanders, who won for the fourth straight time.
Blake Comeau scored on a first-period backhander and Grabner's tally off an assist from Frans Nielsen made it 2-0 after 40 minutes. P.A. Parenteau and James Wisniewski also tallied for the Islanders and Nielsen had two assists.
Tomas Plekanec got the Canadiens' only goal and Carey Price let in four tallies on 25 shots.
The Habs have lost six of their last eight games.