condense has 5 distinct synonyms spread across 3 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
make more concise
“condense the contents of a book into a summary”
cause a gas or vapor to change into a liquid
“The cold air condensed the steam”
become more compact or concentrated
“Her feelings condensed”
develop due to condensation
“All our planets condensed out of the same material”
compress or concentrate
“Congress condensed the three-year plan into a six-month plan”
Synonyms, antonyms and the broader and narrower relations for condense come from Princeton University’s WordNet. Full sources and licences.