The homer, Francoeur's seventh this season, gave New York a 7-1 lead with two outs in the fifth.
Daniel Murphy, Luis Castillo and Omir Santos all had two hits and one RBI, with Santos hitting a solo homer. A run scored on Alex Cora's fourth-inning double play groundout, David Wright hit a solo homer, and Angel Pagan added two hits for the Mets, who stopped their three-game losing streak.
Winner Jonathon Niese (1-0), making sixth major league start, surrendered a run and four hits in seven innings to get the win.
Houston scored on a first-inning RBI single by Ivan Rodriguez and a two-run homer by Hunter Pence. Chris Coste had two hits for the Astros, whose four-game winning streak ended.
The losing pitcher was starter Russ Ortiz (3-5), who went 4 1/3 innings and gave up six runs on five hits with four walks and three strikeouts.