  We theoretically study nonequilibrium spin transport in a superconducting
wire connected by tunnel junctions to two ferromagnetic metal wires, each of
which serves as an injector or detector of spin-polarized electron current. We
present a set of Boltzmann equations to determine nonequilibrium quasiparticle
distributions in this system, and obtain an analytical expression for the
nonlocal spin signal in the case of small injection current. It is shown that
the quasiparticle distribution in the ferromagnetic metal for detection
strongly affects the magnitude of the spin signal. At low temperatures, since
nonequilibrium quasiparticles created by the tunneling from the superconductor
dominate thermally excited ones, the spin signal becomes independent of
temperature. This explains the convergence of the spin signal with decreasing
temperature observed in a recent experiment by Poli \textit{et al}.
