  Coexistence of antiferromagnetism and d-wave superconductivity within a CuO_2
plane was recently observed in a wide doping region for multilayer
high-temperature cuprate superconductors. We find that the experimental phase
diagram is well reproduced in the slave-boson mean-field scheme of the
two-dimensional t-J model by including antiferromagnetic order. We argue that
weak three dimensionality coming from a multilayer structure is sufficient to
stabilize antiferromagnetic order and its coexistence with superconductivity.
