This paper deals with systems of polynomial differential equations, ordinary or with partial derivatives. The embedding theory is the differential algebra of Ritt and Kolchin. We describe an algorithm, named Rosenfeld-Gröbner, which computes a representation for the radical P of the differential ideal generated by any such system S. The computed representation constitutes a normal simplifier for the equivalence relation modulo P (it permits to test membership in P). It permits also to compute Taylor expansions of solutions of S. The algorithm is implemented within a package in MAPLE V.