We present an accurate transfer matrix renormalization group calculation of
the thermodynamics in a quantum spin-1 planar ferromagnetic chain. We also
calculate the field dependence of the magnon gap and confirm the accuracy
of the magnon dispersion derived earlier through a 1/n expansion. We are th
us able to examine the validity of a number of previous calculations and fu
rther analyze a wide range of experiments on CsNiF3 concerning the magnon d
ispersion, magnetization, susceptibility, and specific hear. Although it is
not possible to account for all data with a single set of parameters, the
overall qualitative agreement is good and the remaining discrepancies may r
eflect a departure from ideal quasi-one-dimensional model behavior. Finally
, we present some indirect evidence to the effect that the popular interpre
tation of the excess specific heat in terms of sine-Gordon solitons may not
be appropriate.