X-ray lines have been recently detected in the afterglows of a few gamma-ray bursts. We derive constraints on the physical conditions in the line-emitting gas, using as an example the multiple K$_��$ lines detected by Reeves et al. (2002) in GRB 011211. We argue that models previously discussed in the literature require either a very extreme geometry or too much mass in the line-emitting region. We propose a new model in which gamma-rays and radiation from the early x-ray afterglow are back-scattered by an electron-positron pair screen at a distance of about $10^{14}-10^{15}$ cm from the source and irradiate the expanding outer layers of the supernova ejecta, thereby producing x-ray lines. The model suffers from fewer problems compared to previous models. It also has the advantage of requiring only a single explosion to produce both the GRB and the supernova ejecta, in contrast to most other models for the lines which require the supernova to go off days or weeks prior to the GRB. The model, however, has difficulty explaining the $>10^{48}$ ergs of energy emitted in the x-ray lines, which requires somewhat extreme choices of model parameters. The difficulties associated with the various models are not particular to GRB 011211. They are likely to pose a problem for any GRB with x-ray lines.

19 pages, 1 figure, submitted to Ap. J