Transient heat transfer and water condensation in the engine exhaust system have a strong influence on the conversion of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) in the three-way catalytic converter (TWC) of an engine cold-start. This paper presents the theoretical modeling of heat transfer and chemical reactions of CO, HC, and NO associated with the effects of catalyst surface water vapor condensation, and water evaporation in the entire engine exhaust system fitted with a TWC. The predictive capability of the model is validated successfully with the experimental data.