In entrained coal gasification, the properties of slag and its behavior on the wall are crucial in selecting specific coal types and controlling the gasifier operation. The temperature of critical viscosity (Tcv) is the key parameter that characterizes the slag flowability. This study evaluates the theoretical influences of Tcv, critical viscosity and flux addition on the slag behavior in a commercial coal gasifier using a numerical model. The results showed that a lower slag Tcv value led to an exponential decrease in the solid slag thickness (δS) owing to an increased heat flux to the wall and a reduced temperature difference between the slag and the coolant. In contrast, the influence of Tcv on the liquid slag thickness (δL) was relatively small. When Tcv remained constant, a higher critical viscosity value decreased the velocity of the liquid slag close to the solid slag, leading to a gradual increase in both δS and δL. Addition of flux was effective in decreasing δS for coals with high Tcv values, while the impact of increased slag flow rates on δL was minimal and could be disregarded. The relative magnitude of total slag thickness by changes in Tcv and critical viscosity was almost the same for different ash:flux ratios, gas temperatures, and wall angles. The influence of Tcv was dominant over that of critical viscosity for slags with critical viscosity values above 10 Pa s.