Catalyzes the methyl transfer from S-adenosyl-methionine to the C-24 of zymosterol to form fecosterol. S-adenosyl-L-methionine + zymosterol = fecosterol + H(+) + S-adenosyl-L-homocysteine Steroid metabolism; ergosterol biosynthesis; ergosterol from zymosterol: step 1/5. Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.