TORONTO, June 30 (Reuters) - Canada's main stock index opened lower on Monday as sluggish economic data and weakness in commodity prices weighed on shares of natural resource producers. The Toronto Stock Exchange's S&P/TSX composite index was down 30.86 points, or 0.20 percent, at 15,063.39 shortly after the open. (Reporting by John Tilak; Editing by Chizu Nomiyama)