In high temperature gasification reactors, inert ashes coming with the feedstock deposit on the walls in the form of slag. In these kind of reactors, external cooling screens are used to control slag deposition. A transient 2D model was developed and implemented to simulate the growth of a slag liquid-solid layer at the walls of an entrained flow coal gasifier operating at 1200-1500 degrees C and external cooling at 30-600 degrees C The model results were compared with those of other models present in the literature. The effect of operating variables on layer thickness and wall heat flux were studied through sensitivity analysis. The study shows that the most important parameter controlling slag accumulation is the difference between gas temperature and slag critical viscosity temperature. This affects slag viscosity and eventually the degree of wall agglomeration at steady state. The role of the external cooling temperature and ash flowrate was also investigated. The model was modified to account for the effect of gas cooling due to wall heat transfer by convection and radiation. Results show that if gas flowrate is low, e.g. a small-scale unit, this effect is not negligible, contrarily to what is usually assumed in the literature. (C) 2017 Elsevier Ltd. All rights reserved.