Catalyzes the first and rate-limiting reaction of the four reactions that constitute the long-chain fatty acids elongation cycle. Uses malonyl-CoA to add 2 carbons per cycle to the chain of long-chain fatty acids. Condensing enzyme that catalyzes the elongation of monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. (6Z,9Z,12Z)-octadecatrienoyl-CoA + H(+) + malonyl-CoA = (8Z,11Z,14Z)-3-oxoeicosatrienoyl-CoA + CO2 + CoA (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA + H(+) + malonyl-CoA = (8Z,11Z,14Z,17Z)-3-oxoicosatetraenoyl-CoA + CO2 + CoA (9Z)-hexadecenoyl-CoA + H(+) + malonyl-CoA = 3-oxo-(11Z)-octadecenoyl-CoA + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Belongs to the ELO family.