  We present the addition of nebular emission from the narrow-line regions
(NLR) surrounding active galactic nuclei (AGN) to BEAGLE (BayEsian Analysis of
GaLaxy sEds). Using a set of idealised spectra, we fit to a set of observables
(emission-line ratios and fluxes) and test the retrieval of different physical
parameters. We find that fitting to standard diagnostic-line ratios from
Baldwin et al. (1981) plus [O II]3726,3729/[O III]5007, Hbeta/ Halpha, [O
I]6300/[O II]3726,3729 and Halpha flux, degeneracies remain between
dust-to-metal mass ratio and ionisation parameter in the NLR gas, and between
slope of the ionizing radiation (characterising the emission from the accretion
disc around the central black hole) and total accretion-disc luminosity. Since
these degeneracies bias the retrieval of other parameters even at maximal
signal-to-noise ratio (S/N), without additional observables, we suggest fixing
the slope of the ionizing radiation and dust-to-metal mass ratios in both NLR
and HII regions. We explore the S/N in Hbeta required for un-biased estimates
of physical parameters, finding that S/N(Hbeta)~10 is sufficient to identify a
NLR contribution, but that higher S/N is required for un-biased parameter
retrieval (~20 for NLR-dominated systems, ~sim30 for objects with
approximately-equal Hbeta contributions from NLR and HII regions). We also
compare the predictions of our models for different line ratios to
previously-published models and data. By adding [He II]4686-line measurements
to a set of published line fluxes for a sample of 463 AGN NLR, we show that our
models with $-4<$ionisation parameter in the NLR gas$<-1.5$ can account for the
full range of observed AGN properties in the local Universe.
