  This and companion papers by Harrington et al. and Blecic et al. present the
Bayesian Atmospheric Radiative Transfer ({\BART}) code, an open-source,
open-development package to characterize extrasolar-planet atmospheres. {\BART}
combines a thermochemical equilibrium abundances ({\TEA}), a radiative-transfer
({\transit}), and a Bayesian statistical (MC3) module to constrain atmospheric
temperatures and molecular abundances for given spectroscopic observations.
Here, we describe the {\transit} radiative-transfer package, an efficient
line-by-line radiative-transfer C code for one-dimensional atmospheres,
developed by P. Rojo and further modified by the UCF exoplanet group. This code
produces transmission and hemisphere-integrated emission spectra. {\transit}
handles line-by-line opacities from HITRAN, Partridge \& Schwenke ({\water}),
Schwenke (TiO), and Plez (VO); and collision-induced absorption from Borysow,
HITRAN, and ExoMol. {\transit} emission-spectra models agree with models from
C. Morley (priv. comm.) within a few percent. We applied {\BART} to the
{\Spitzer} and {\Hubble} transit observations of the Neptune-sized planet
HAT-P-11b. Our results generally agree with those from previous studies,
constraining the {\water} abundance and finding an atmosphere enhanced in heavy
elements. Different conclusions start to emerge when we make different
assumptions from other studies. The {\BART} source code and documentation are
available at https://github.com/exosports/BART.
