A mathematical model, including energy balances and thermal effects, for the polymerization of ethylene using silica-supported chromium catalysts is presented. The model is based on a previous work, with the main feature being its structural nature. Catalyst characterization and morphology data are used directly in a physical scheme that does not require the assumptions concerning particle geometry found in other models in the literature. Predictions are made using the model for the cases of high activity catalysts under experimental (laboratory) and industrial reactor conditions. Catalyst fragmentation is modeled as a set of steps that follow the actual rupture process of the initial solid into fragments with irregular shapes. Parameters are evaluated from experimental data for the initial support. From modeling results, mass transfer control is not observed. Neither external mass transport phenomena, nor internal diffusive processes become restrictive for polymerization although monomer concentration decay exists. Temperature rise is relevant because of the high catalyst activity and monomer concentration typical of industrial systems. However, under controlled polymerization conditions, temperature elevation is important but not critical to polymer production. The limiting step in the energy transport process in the particle is the external transfer. The results obtained show a strong influence of reactor conditions upon polymerization reaction and polymer properties, and are in good agreement with the available data.