This work is devoted to the development and verification of a new intrinsic-based subgrid model for moving char particles gasifying in a hot flue gas or syngas environments consisting of CO2/H2O/CO species. The distinguishing feature of our model relative to the submodels published in the literature is that it takes into account the thermal and chemical nonequilibrium between the particle's surface and its center. Thus, our model is able to predict temperature and species gradients inside the particles. The main focus of the new submodel is to demonstrate the crucial role of intrinsic-based heterogeneous reactions in the adequate prediction of carbon conversion rates for char particles gasification in fixed-bed and fluidized-bed gasifiers. The new model is verified against steady-state, particle-resolved computational fluid dynamics (CFD)-based, three-dimensional simulations carried out for different volume fractions of solid phase in a control volume (CV). Acceptable agreement has been demonstrated. Finally, to demonstrate our new model's predictions, we carried out several unsteady simulations for different ambient temperatures and Reynolds numbers. The importance of simultaneous change of char porosity and particles size during gasification has been demonstrated for different regimes indicated by the Damköhler numbers.