An isothermal model, incorporating the two-phase theory, has been developed to evaluate the performance of a bubbling fluidised-bed coal gasifier. A distinctive feature of this model is the consideration of a net flow term from the emulsion phase to the bubble phase in the conservation equations. Simulations with consideration of the net flow term indicate that the overall results compare favourably with available experimental data from an industrial fluidised-bed gasifier reported in the literature. The net flow is significant, in the range 71-87% relative to the feed gas rate, strongly depending on the coal rank, heterogeneous reaction rates and volatile matter released in the bed. The higher the coal rank, the lower the net flow and total excess Pas flow. The large volume of net flow generated can significantly change the fluidisation conditions in the bed and thus alter the reaction rates and mass transfer properties. Simulations without the net flow deviate significantly from the experimental results.