How to do Hexen-style level transitions (as in make it so that you aren't starting over if you back to the same map)?

[Blast98]

So, I need to figure out to make map transitions that are like that of what the levels in Hexen because I'm trying to work on a Hexen wad (even though I'm also working on Doom wads) and I want to know how to make the level transitions so that you aren't starting over if you teleport to the same map in a hub and instead, the level looks the same as before you exited it (the monsters you killed are killed, the items you picked up are picked up, and so on). If anyone knows how this method of level transition works, I would appreciate it. 

[DankMetal]

Chubzdoomer has a mapinfo tutorial, in the final section he shows how to make that style of hubs.