The cooling of polypropylene melts and their solidification is analyzed taking into account the mutual dependency of crystallization and heat transfer. A single equation for the crystallinity is used. This equation also provides information for the ensuing microstructure. No restrictive assumption is necessary when nucleation kinematics and growth rates are known. Crystallization itself has a strong influence on heat transfer and cooling time during processing because all material parameters depend on crystallinity. The theoretical results obtained with these parameters compare favorably with the temperature drop and the increase of crystallinity observed during cooling experiments. The potential of a simple method for evaluating the spherulitic structure at the end of crystallization is demonstrated by comparing the calculated and the measured size distribution of spherulites in a solidified slab. The combined computation of heat transfer and crystallization presented in this study is important for calculating the cooling time of plastics processing operations. POLYM. ENG. SCI., 56:1291-1302, 2016. (c) 2016 Society of Plastics Engineers