Vokoun finished with 30 saves, including 10 in each of the game's three periods, racking up his 21st victory.
Semin's goal 12 minutes into the final period capped the scoring for Washington, which also got tallies from Dennis Wideman and Matt Hendricks in the victory.
It was the Capitals' first road win since Jan. 18, when they also shut out the Canadiens 3-0.
Peter Budaj couldn't prevent the Habs' third straight win despite making 20 saves.
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