The segregation of the low-molecular-weight fractions on the crystal growth of polydisperse polymers has been identified as a general phenomenon by means of dynamic Monte Carlo simulations of lattice polymer systems. Its mechanism was elucidated on the basis of the intramolecular nucleation model. A unified scheme has been proposed to interpret all three scenarios of molecular segregation according to the location of the crystallization temperature T-c demarcated by two melting points of short-chain fractions, namely T-m(0) for the bulk extended-chain crystals and T-m2D for two-dimensional single-folded-chain crystals on a smooth crystal-growth front. In the first scenario, T-c > T-m(0), and short-chain fractions are thermodynamically forbidden in crystallization, like the small solvents in conventional monotectic polymer solutions. In the second scenario, T-m(0) > T-c > T-m2D, and short-chain fractions are fully excluded by their failures on the intramolecular secondary nucleation for folded-chain crystal growth. In the third scenario, T-c less than or similar to T-m2D, and short-chain fractions are partially segregated due to their much less free energy gains in the intramolecular secondary nucleation than long-chain fractions. When T-c << T-m2D, both short-chain and long-chain fractions cocrystallize without any molecular segregation. In addition, an upper limit of molecular weights in crystallization fractionation has been explained.