Purpose Underground coal gasification is a very difficult process due to changes in the parameters over time and within the space of the reactor and a variety of phenomena that occurs there. Consequently, it is necessary to create appropriate mathemati-cal models, which simplify the optimization and forecast of future results (especially final gas properties). The purpose of this work was the detailed critical analysis of the simplest coal gasification simulations methods, based on thermodynamic calculations of the process. These models, called equilibrium models, can be divided into two groups: stoichiometric and non-stoichiometric. The other aim of this paper was to characterize various engineering methods used in the calculation of equilibrium gasification processes. Methods Literary studies, concerned with general aspects of underground coal gasification modeling and the modeling of coal gasi-fication in the manner of equilibrium calculations, were used as a research method applied in presented work. Results The critical analysis of equilibrium models of coal gasification and the characterization (including mathematical formulation of process, range of required parameters, rate of convergence of calculations and methodology of searching solutions) of stoichiometric and non-stoichiometric methods were results of numerous considerations presented in this article. Practical implications The work presented describes practical issues connected with equilibrium models – their advantages, limitations and possible problems (for example with the determination of required constants) and potential applications (preliminary analysis, point of reference to more complex simulations etc.). Originality/ value This paper presents state of the art in field of equilibrium coal gasification modeling. This article is also attempt to elaborate on the most important problems connected with thermodynamic models of coal gasification.