See the following 2008 article for how the "world's leading" GtL company is going with its large scale GtL plant.
http://www.engineeringnews.co.za/article.php?a_id=128827
And this is GtL not combined with a UCG process which wil be much more difficult than using natural gas!
Not saying UCG-GtL can't be done - probably can be - but don't think this will be a picnic.