A rigorous rate-based model for CO2 absorption using aqueous ammonia in a packed column has been developed and used to simulate results from a recent pilot plant trial of an aqueous ammonia-based post-combustion capture process at the Munmorah Power Station, New South Wales, Australia. The model is based on the RateSep module, a rate-based absorption and stripping unit operation model in Aspen Plus, and uses the available thermodynamic, kinetic and transport property models for the NH3–CO2–H2O system to predict the performance of CO2 capture. The thermodynamic and transport property models satisfactorily predict experimental results from the published literature. The modelling results from the rate-based model also agree reasonably well with pilot plant results, including CO2 absorption rate, NH3 loss rate, temperature profiles and mass transfer coefficients in the absorber. To gain insights into absorption performance, we used the rate-based model to analyse the species concentration profile, temperature profile, mass transfer rate and coefficient in the gas and liquid bulk phase along the packing height.