A model has been formulated to describe the quasi-steady-state gasification of coal-derived chars in gas mixtures where both the reactants carbon dioxide and steam, and the gasification products carbon monoxide and hydrogen are present. As such, these conditions reflect the situation found in most practical gasification systems. The model presented is applied under conditions where intraparticle mass transfer is rate-controlling. Intraparticle heat transfer is neglected. In view of the non-equimolar gasification reactions, the mass flux equations are derived from the continuum limit of the dusty gas model. These flux equations are combined with strongly non-linear Langmuir-Hinshelwood kinetics for the gasification reactions. The model accounts for local variations of the diffusive and convective permeability parameters, as well as variations in the reactive surface area, during burnoff of a char particle. The impact of the various relevant mass-transfer parameters, the gasification temperature and pressure, and the char particle size on the gasification behaviour is discussed. A comparison is made between the present model and existing models, and various modelling approaches are critically reviewed.