The striker, who was signed by Iain Dowie, has not made a first team appearance since joining from Charlton and was loaned to Kilmarnock in August, due to City not having a reserve team this season.
Simmonds, who has an ankle injury, is to have an hernia operation next week and this will mean the end of his spell at Rugby Park.
When he regains match fitness, he is expected to be loan back out again in the hope of him winning a contract elsewhere.