Catalyzes the methyl transfer from S-adenosyl-methionine to the C-24 of cycloartenol to form 24-methylene cycloartenol. S-adenosyl-L-methionine + zymosterol = fecosterol + H(+) + S-adenosyl-L-homocysteine Steroid biosynthesis; sterol biosynthesis. Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.