composed has no recorded synonym. 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. Its one recorded opposite is discomposed.
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.
serenely self-possessed and free from agitation especially in times of stress
“the performer seemed completely composed as she stepped onto the stage”
Synonyms, antonyms and the broader and narrower relations for composed come from Princeton University’s WordNet. Full sources and licences.