Many industrial processes like ammonia and methanol syntheses, SO//2 oxidation and vapor-phase hydrogenations, involve the decrease in the total number of moles, which creates an inward flow in the pores of the catalyst. For the product to be able to diffuse against this flow, high concentration gradient is needed; otherwise, the rate of process slows down. This equilibrium conditions can be reached inside the pores, while under the bulk gas conditions outside the pores, the system is far from equilibrium limitations. This is the reason why diffusional limitations can be experimentally observed under certain conditions where the Weisz-Prater criteria would predict none. The phenomenon was mathematically modeled and digitally simulated for a case of multiple reactions under non-isothermal conditions.