The design of in situ gasification systems for coals that soften on heating is considered. It is shown that these coals will not support open channels against overburden pressure for any significant length of time. Thus, the zones of heated and softened coal must be localized. This study is applicable to some subbituminous as well as bituminous coals. A workable gasification system for these coals would involve flow through widely spaced channels typical of the Soviet gasification systems. To determine how widely spaced the channels should be, the heat wave propagation depth from the channel into the coal must first be calculated. It is shown that if the coal surface in the reaction zone is burning at 1 m/d, heat is carried by conduction only 30 mm into the coal. This limited conductive heat propagation explains why reverse combustion processes tend to create narrow channels. However, downstream of the reaction zone, channel heating by product gases can propagate into the coal. For the typical Soviet line-drive system, heating will propagate to a depth no more than 18% of the distance between channels as the gasification of a section is completed. Hence, channel closing caused by coal softening has only a small effect in this system.