The migration rates of representative underground coal gasification (UCG) related products and contaminants through porous surroundings of a georeactor in a shallow coal seam in gaseous and liquid phases were studied in this work. The proposed comprehensive model, taking into account mass transport phenomena both in gaseous and liquid phase, was based on the real data obtained during the UCG field-scale study carried out at the Experimental Mine “Barbara” in Poland in the framework of the RFCS Project HUGE (2007–2010). The mass transport from the reaction zone was modeled by means of constitutive equations, in gas phase permeation, in liquid phase advection, axial dispersion, diffusion, and adsorption. The applied input values of parameters were based on results from previous pilot experiments. The effects of pore size, porosity, tortuosity, and reaction conditions on the migration rate were individually evaluated and taken into account. The contamination risk for aquifers and potential leakage of poisonous gases and their environmental impact were assessed and thoroughly discussed. It was found that porosity, pore diameter and tortuosity, which belong to the key transport parameters, play a major role in overall permeation transport of gaseous contaminants compared with physical characteristics—pressure and temperature. Migration rates of UCG-related contaminants in the liquid phase are significantly slower compared with the gaseous phase.