Reimer made 12 saves in the first period as the Leafs were scoring four goals to grab a big early lead.
Davis Drewiske scored for the Canadiens, who remain one point ahead of Boston for the Northwest Division lead.
Carey Price started in goal but was pulled in the initial frame after yielding three goals on just four shots.
Peter Budaj relieved and allowed two goals on 24 shots the rest of the way.
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