Catalyzes the methyl transfer from S-adenosyl-methionine to the C-24 of cycloartenol to form 24-methylene cycloartenol. cycloartenol + S-adenosyl-L-methionine = 24-methylenecycloartanol + H(+) + S-adenosyl-L-homocysteine Steroid biosynthesis; sterol biosynthesis. Highly expressed in vascular tissue, mature leaves and in regions undergoing cellular expansion. Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.