This article presents a novel approach to treat heterogeneous catalytic reactions occurring in porous or honeycomb monoliths. The approach allows accurate modeling of full-scale catalytic converters with low computational cost. In this approach, the entire catalytic monolith is treated as an anisotropjc porous medium, and sub-grid scale models are employed to represent the heterogeneous chemical reactions occurring at the solid-fluid interfaces within the monolith. Full coupling between fluid flow, heat transfer, species transport, and heterogeneous chemical reactions is achieved through flux balance of species and energy at the solid-fluid interfaces. The model allows for unlimited number of finite-rate reaction steps and species, including surface-adsorbed species and site coverage effects. The model was validated for hydrogen-assisted combustion of methane-air mixtures over platinum catalyst clusters in a full-scale catalytic converter. Validation against experimental data exhibits excellent match for ignition temperature for various methane and/or hydrogen inlet concentrations. Transient calculations show that the time constant for ignition matches well with previously reported results. Because the model is based on a sub-grid scale approach, it is orders of magnitude more efficient for modeling full-scale catalytic converters than conventional approaches where each channel within the catalytic monolith has to be represented by a computational grid.