A rigorous one-dimensional heterogeneous model is developed for a secondary reformer in an ammonia plant with special emphasis on the catalyst particle models. These are based on the effective diffusivity model and the Stefan-Maxwell model in the catalyst pore and the film around the catalyst particle. The performance of these four models is evaluated for the data collected from a commercial reformer in the complete operating range and recommendations are made regarding the use of appropriate particle models. The models for the catalyst particle show considerable deviations at the particle level but the reformer simulations produce almost identical results at the global level for all the four particle models. The need for a rigorous treatment of interphase resistance to mass transfer has been clearly demonstrated by comparison of the present model with the model of Singh and Saraf.