This work is devoted to the development and validation of a new 0-D model (sub-model) for the steam reforming of methane inside a spherical catalyst particle (Ni/α-Al2O3) placed in a hot methane/steam atmosphere. The particle diameter is 4 mm. The submodel includes six gaseous chemical species (CH4, CO2, CO, H2O, H2, N2) and uses experimentally defined reaction rate expressions taken from the literature. The distinguishing feature of the subgrid model is its ability to take into account the internal heat and mass transfer coupled with the heat and mass transfer on the catalyst surface influenced by convection and diffusion around the particle. The sub-model was validated against a comprehensive 3D-CFD-based model resolving the issues of bulk flow, thermal and species boundary layers around the particle and convection and diffusion processes inside the porous catalyst particle. Good agreement was achieved between the new 0-D model and the 3D CFD-based model.