A coal gasification simulator has been being developed to analyse and predict thermal, chemical, and physical phenomena in pressurized entrained-flow coal gasifiers. The simulator, when completed, will be able to evaluate performance in different types of gasifiers and/or design of such gasifiers. Devolatilisation, char gasification reactions and ash particle deposition on the walls occur simultaneously in slag-tap coal gasification reactors. This paper focuses on ash deposition with an ash deposition submodel that uses the ash viscosity as the criteria for slagging. It has been incorporated into the modern computational fluid dynamic code "FLUENT", and the solutions were then compared with the data measured in a 50 ton/day entrained flow coal gasification pilot plant, called HYCOL. We found that the modified "FLUENT" can predict ash deposition profiles fairly accurately.