An in situ coal gasification process adapted for large scale commercial projects is provided. Techniques are provided to insure establishment of a gasification front over the full seam thickness as each successive injection well in the array is brought on line. This is accomplished by controlling the oxidant introduction in a prescribed manner during the early stages of injection after pneumatic communication between well pairs has been established. Also provided are techniques and standards for avoiding or controlling subsidence and for conducting gasification operations in free water laden seams and in coal seams subject to spontaneous combustion.