A mathematical model for the isothermal, slurry polymerization of ethylene using solid Ziegler-Natta catalysts is developed, accounting for the effect of gas-liquid mass transfer limitations on the overall rate and polymer properties, such as molecular weight and polydispersity index. The existence of micron-sized catalyst particles in the initial stages of polymerization influences the final polymer properties and also leads to the diffusion and reaction steps being in parallel rather than in series. The gas-liquid interfacial contribution to the polymer formation is shown to significantly enhance the value of the polydispersity index. Higbie’s penetration theory has been satisfactorily employed to model the monomer absorption in the presence of growing polymer macroparticles. High polydispersities are shown to arise in the presence of gas-liquid mass transfer limitations, even when only one type of active catalyst site is considered, and when the intra-macroparticular diffusional resistance is low.