Impact of mass and heat transfer on exothermic reaction performance in porous catalytic monolith with triangular channels is investigated by CFD modeling. Detailed analysis of spatial distributions of process characteristics for methane oxidation shows that in the initial part of catalyst volume there is the active subsurface layer. The domain of sharp gradients of the reaction rate is revealed that includes the parts of external surface and thin subsurface layers near the monolith inlet, which results in sharp rearrangement of 3D-field of temperature and reagent concentrations. It is shown that the formation of such conditions is strongly influenced by complex gaseous flow reconstruction with gas penetration into the catalyst volume, significant heat release, and heat transfer between channel wall and gas flow. Though the region with high reaction rate is rather short this could be of high importance for reactor design and selection of optimal operation conditions.