Samsung Galaxy S4 mini — Galaxy S4 Mini overheats.

Hello
I am new to the forum. I just hope I am doing it the proper way
I have a Samsung S4 Mini since a few years now. A few weeks ago, it started overheating and the battery was discharging very quickly despite on charge.
Then it switched off by itself. I found out the battery was dead. I bought a new battery. Then as soon as I started to charge it was already overheating. I switched it off immediately and it continued to overheat. Yes even switched off it was still overheating. I had to remove the battery to stop the overheating. Any one else had this issue. I tried to google "Phone overheating even if switched off" but no result at all.
Now I need to get it fixed, guarantee is long gone. What parts you think needs to be replaced? Thanks for help
Cheers
Laval