The problem of dissipation of latent heat of fusion by thermal conductivity in polymer crystallizing in bulk is discussed. It is shown that the liberation of latent heat of fusion causes the increase of the temperature at the crystallization front. The increase of thermal conductivity and diffusivity of a polymer due to phase transition decreases while the presence of another crystallization front increases the temperature elevation. The temperature buildup is limited by the decrease of growth rate with temperature, directly related to the release of latent heat of fusion. The thermal effects due to crystallization of numerous spherulites in a plate are also estimated. It is shown that the crystallization causes a significant increase of the temperature inside the material as the crystallization proceeds, which slows the rate of conversion of melt into spherulites. The temperature increase in the interior of material depends on the initial temperature and on the sample sizes. The predictions were confirmed by the measurements of the temperature in bulk samples of isotactic polypropylene during crystallization at isothermal ambient conditions.