A density-functional theory for a polyatomic system is applied to gas-liquid nucleation in n-butane and n-heptane, employing an interaction site model and the rotational isomeric state approximation. Effects of chain length and flexibility on equilibrium properties and nucleation are discussed. It is shown that the n-alkane systems cannot be well approximated by a system with the spherically symmetric Lennard-Jones potential, giving a nucleation rate scaled by the classical rate smaller by six orders of magnitude.