The underground coal gasification (UCG) process represents a modern and effective coal mining technology that enables coal energy extraction through thermic decomposition. The coal is transformed into syngas by oxidizers (e.g., air, technical oxygen, or water steam) and is injected into a georeactor. The produced syngas is exhausted on the surface, where it is transformed into the desired form of energy. This paper presents an experimental study of two experiments performed in ex-situ reactors. The paper describes the equipment for the UCG process, the physical models of the coal seam, and the analysis of coal. The obtained results from the experiments are presented as the behavior of the temperatures in the coal during the experiment, the syngas composition, and its calorific value. The material balance and effective gasification time of the UCG process were also identified for the individual experiments. The aim was to evaluate the impact of the coal seam model on the gasification process efficiency. Calculating the material balance during the gasification appears to be an effective tool for assessing leaks in the reactor while measuring the flow and concentration of the oxidizers and produced gas. The material balance data are make it possible to propose methods for controlling the input oxidizers. To increase the efficiency of the gasification in an ex-situ reactor, it is necessary to ensure the impermeable or poorly permeable surrounding layers of the coal seam.