What is the IUPAC systematic name for methane? 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"?
The latest edition of the Blue Book, the 2013 Revision, clarifies this:


  Methane is a retained name (see P-12.3) that is preferred to the systematic name ‘carbane’, a name never recommended to replace methane, but used to derive the names ‘carbene’ and carbyne for the radicals $\ce{H2C^2-}$ and $\ce{HC^3-}$, respectively.


So it seems to be the case that the systematic IUPAC name is only there for consistency with the ions, and is not recommended (and not used) to describe methane. Similarly, "ethane", "propane", and "butane" are preferred to their systematic equivalents "dicarbane", "tricarbane", and "tetracarbane".