  Context - Exoplanetary upper atmospheres are low density environments where
radiative processes can compete with collisional ones and introduce non-local
thermodynamic equilibrium (NLTE) effects into transmission spectra.
  Aims - We develop a NLTE radiative transfer framework capable of modelling
exoplanetary transmission spectra over a wide range of planetary properties.
  Methods - We adapt the NLTE spectral synthesis code Cloudy to produce an
atmospheric structure and atomic transmission spectrum in both NLTE and local
thermodynamic equilibrium (LTE) for the hot Jupiter HD209458b, given a
published T-P profile and assuming solar metallicity. Selected spectral
features, including H$\alpha$, Na I D, He I $\lambda$10830, Fe I & II
ultra-violet (UV) bands, and C, O and Si UV lines, are compared with literature
observations and models where available. The strength of NLTE effects are
measured for individual spectral lines to identify which features are most
strongly affected.
  Results - The developed modelling framework computing NLTE synthetic spectra
reproduces literature results for the He I $\lambda$10830 triplet, the Na I D
lines, and the forest of Fe I lines in the optical. Individual spectral lines
in the NLTE spectrum exhibit up to 40 % stronger absorption relative to the LTE
spectrum.
