In this paper, a new algorithm computing the power flow for multi-area power systems is presented. This algorithm is suitable for synchronous areas interconnected by means of AC tie-lines. In particular, a new iterative composition/decomposition matrix procedure is adopted. For each control area, the classical PV, PQ and slack bus constraints are defined, hence the power flow solutions of each control area can be computed independently. In case, this independence allows exploiting the parallel computation technique. The overall power flow is then computed by putting together all the solutions of each control area iteratively, by means of the tie-line (i.e., the lines interconnecting the areas) admittance matrix. The present multi-area method is completely general and once the power flow solution of each area is separately achieved by any power flow solver (e.g., Newton-Raphson and derived, PFPD, or other), it makes suitable use of both a Thevenin's theorem generalization and a novel tie-line admittance matrix. In this direction, the method is not a new power flow algorithm but a new multi-area one which starts from the solutions of the power flow of area, each of that with its own slack-bus. Applications of the algorithm to standard test cases are presented. Eventually, to test the validity of the method, numerical comparisons with the commercial software DIgSILENT PowerFactory are performed.