A mathematical model is developed for a moving bed type coke gasifier based on the theories of transport phenomena, chemical reaction kinetics and two-phase flow. This model consists of conservation equations of momentum, heat and mass, and equation of continuity for reaction gas and coke particles together with reaction rate equations. The model estimates temperature and concentration distributions of gas and solid within the gasifier as well as flow characteristics. The validity of this model is confirmed by comparison of measured and computed temperature distributions in the gasifier. Numerical experiments, which were performed to study the effect of furnace diameter, feed gas composition and feed rate of gasification agents, clarify the effect of these parameters on the furnace operation. Additionally, computation results show the importance of heat loss through the furnace wall for a gasifier with a water jacket.