When polyolefins are produced on heterogeneous catalysts, they encapsulate the catalyst fragments and present diffusional resistance to further monomer transport to the catalyst fragments. In addition, the deposited polymer layer brings in viscoelastic resistance, since it must be deformed to make space for the polymer being formed at the catalyst surface. In the present work, we analyze the influence of the growing polymer layer on the development of the stress and the concentration profiles and, hence, on the polymerization rate.