Coal gasification is very polluting. It requires (at least the early methods) lots of steam (ie water), and energy (either coal or electric in Germany's case) to produce "coal gas." Now, also note that most of the fuel produced is short chain hydrocarbons like methane and are not suitable for use as vehicle fuels directly.
The other problem is what to do with waste products like carbon dioxide, sulfides, and slag / fly ash.
The US (among others) is still experimenting with methods to cleanly commercialize this process. But, it is still only marginally viable compared to oil drilling and production.