Commander Keen in Goodbye, Galaxy was released for MS-DOS on December 15, 1991, as the second main game in the series. Consisting of episodes four and five of the series, "Secret of the Oracle" and "The Armageddon Machine", and stylized as Commander Keen in Goodbye, Galaxy!, it was developed by id and published through Apogee. Like the first trilogy of episodes, "Secret of the Oracle" was released for free, and the other episode sold by Apogee. The game incorporates a more polished version of the graphical updates in Keen Dreams, along with further gameplay changes and additions.
Source: Wikipedia