A computational fluid dynamics software package was used to study heat transfer from spherical particles of different sizes and under different heat-transfer conditions. It was shown that although the Ranz-Marshall and other similar correlations are valid in the case where particles do not interact, this is not true for densely packed systems such as those that we find in reactors commonly used in olefin polymerization. It,, ns also demonstrated that convection is in fact not the only means of removing heat from small highly? active particles. Conductive heat transfer between large and small particles present in the same reactor appears to help alleviate problems of overheating and explain why earlier models of heat transfer in olefin polymerization overpredict the temperature rise during early stages of polymerization.