Since this word is made up of a single kanji, it would normally use the kun'yomi reading. The kanji that makes this word only has one important reading, though, and that happens to be the on'yomi. Since you learned the on'yomi reading for this kanji, you've already learned the reading for this word.
Pochi became a star in heaven.
God in Heaven definitely saw me steal Koichi's 4000 yen.
I won the rock-paper-scissors competition and I felt like I was in heaven.