Patching and Plastering - repairing plaster (not drywall) walls

I recently purchased a 1940's bungalow with plaster walls and original floor heaters. I had professionals remove the floor heaters and install central a/c. They did a "rough finish" on the walls, but I'm left with a large square patch of drywall that's roughly 1/2" away from the rest of the plastered wall.

Can anyone suggest how best to make the wall smooth again? Do I just have to layer plaster on top of plaster untill I get to the necessary thickness? How can I make it appear to be the same as the rest of the wall?

2 ways to deal with it. first you can rip out their patch and re do it. You need to shim the wall out so the drywall will sit flush with the rest of the wall. or you can fill in the space with mud until flush. Once you have it flush you need to tape (with mesh) the entire patched area. Then finish the patch with layers of drywall mud until flat and smooth. Do you have texture on your walls?

Thanks for the post. Unfortunately my walls are smooth, not textured, so it's a little more of a challenge!