A dynamic model for the molecular weight properties of polymers with potential gel formation is constructed based on the "Numerical Fractionation" technique. This technique, introduced earlier, relies on the numerical segregation of the polymer population into a series of unimodal subdistributions of similar structure and size. The concepts leading to its derivation provide a new framework in which gelation can be explained. The phenomena accompanying the molecular weight divergence at the gel point are found to be related to heterogeneities in the polymer microstructure. The method presented allows for the computation of sol properties, gel fraction, the gel point, and the reconstruction of the molecular weight distribution. The discontinuity in molecular weight moments at the gel point is avoided by this method, and thus continuous integration through the gel point is possible. Sol properties in the postgel regime can also be computed since the definition of polymer generations allows for the numerical separation of sol and gel at all times. Finally, the method provides a criterion for a priori determination of the potential for gelation of any specific reaction mechanism.