How can I reinstall Safari back to my iMac?

You have to work at removing required macOS components so I suspect you may simply have misplaced its icon. Search for it with Spotlight: Search with Spotlight on Mac - Apple Support

However, as lkrupp and Ronasara wrote if Safari is really gone, the only way to get it back is to reinstall macOS or restore a Time Machine backup that predates its removal.

Reinstall macOS - Apple Support

Recover all your files from a Time Machine backup - Apple Support

It also raises the question what else may have been removed from your Mac.

As has been pointed out to you the user cannot delete or uninstall Safari as it’s part of macOS. You may have only deleted the icon in the dock which his not the app but simply a pointer or alias to the app. Look in your Applications folder or hold down command+space and type in Safari.

For the record, there is no standalone installer for Safari. If you somehow, someway did manage (highly unlikely unless you are a Unix guru) to uninstall it your only recourse it to reinstall macOS entirely to get it back.

I agree with Ikrupp's comments. If you open the Applications folder and locate the Safari there, you can just drag it down to the dock and that action will create a Safari alias there for you to access.

Safari itself cannot be uninstalled. If you can’t open a website in Safari on Mac - Apple Support has a number of suggestions to follow.