The 1993 IUPAC Blue Book claims that "methane" is a semisystematic name:
Semisystematic names also exist, such as "methane", "propanol", and "benzoic acid", which are so familiar that few chemists realize that they are not fully systematic. They are retained, and indeed, in some cases there are no better systematic alternatives.
Propan-1-ol and phenylmethanoic acid are obvious systematic versions of propanol and benzoic acid. (Though I disagree that they are "better".)
What is the "fully systematic" name for methane—or is this a case where there is no "better systematic alternative"?