Temperature rise in a polypropylene particle provoked by the heat of reaction during the gas phase polymerization of propylene over a heterogeneous catalyst was investigated. When the polypropylene particle was approximated to have multigrain morphology, and when mass transfer resistance of the monomer was assumed to be negligible, the temperature within the polymer particle was predicted to increase very sharply to bring about melting of the polymer. Prepolymerization was shown to be effective in reducing the temperature rise to minimize the loss of the stereoregularity controlling ability of the catalyst.