  Recent experiments on the quasi-1D antiferromagnets $R_{2}BaNiO_{5}$ (R =
rare earth) have shown the existence of purely 1D Haldane gap excitations
propagating on the Ni chains. Below an ordering temperature, the gap
excitations survive and coexist with the conventional spin waves in the ordered
phase. We construct a model mixed-spin system in 2D for which the ground state
can be exactly specified. Using the Matrix Product Method, we show the
existence of Haldane gap excitations in the ordered phase. We consider
different cases of ordering to study the effect of ordering on the degeneracy
of the Haldane gap excitations.
