Some respondents imply that steam or water will only remove tiles that were stuck on with wallpaper paste or something similar.
My house had some poly tiles stuck to one ceiling. I removed the bulk of them mechanically with a paint scraper. Each tile was stuck to the ceiling with five blobs of glue. The type of glue was unknown, but I would assume it was something sold as a poly tile adhesive.
I was delighted to discover that a steam wallpaper stripper rapidly softened the adhesive and it was possible to entirely remove it with a paint scraper.
Once the glue was removed, it only took a couple of coats of paint to produce a flawless ceiling.
I had initially been resigned to replacing or re-plastering that ceiling, but was surprised at just how cleanly the adhesive could be removed.