A comprehensive mathematical particle growth model, accounting for both external and internal mass and heat transfer limitations appearing during the early growth of highly active Ziegler-Natta catalysts in gas-phase olefin polymerizations, is developed. Based on the well-known "Polymeric Flow Model" approximation for particle growth [see Schmeal and Street (A.I.Ch.E. Journal 17 (1971) 1188) and Galvan and Tirrell (Chemical Engineering Science 41 (1986) 2385)], dynamic mass and energy balances, including both diffusional and convective mass and heat fluxes, are derived to describe the spatial-time evolution of monomer concentration and temperature in a growing polymer particle. To calculate the equilibrium monomer concentration in the amorphous polymer phase, the Sanchez-Lacombe (Macromolecules 11 (1978) 1145) equation of state is employed. The effect of effective monomer diffusivity, initial catalyst size and catalyst activity on the overall particle growth rate and particle overheating are analyzed in detail. It is shown that, during the early growth of highly active catalyst particles (e.g., polymer production rates > 15,000 g/g(cat) h), internal and external mass and heat transfer resistances can become significant, leading to catastrophic particle burn. In an attempt to reduce overheating of highly active catalyst particles, a prepolymerization step is introduced to control catalyst activity and, thus, particle morphology during the early growth of catalyst particles. The effect of prepolymerization time and temperature on the overall polymerization rate and particle overheating is investigated. It is demonstrated that by proper selection of prepolymerization temperature and time, particle overheating can significantly be reduced while the polymerization rate is enhanced.