Load-bearing beam replacement..with out the walls above coming down
 

My wife and I bought our first house 2 years ago in Lititz P.A. and its about 120 years old. Its a fixer-uper and we knew that but now its looking like more of a Falling-Downer. Recently we discovered we have powder post beetles as well as brown cubical wood rot. They both reeked havic on my house..I'm in the process of replacing a few floor joists now. In the back of my mind I know I HAVE to replace the Load-bearing beam.
How do i do this with out the walls above falling?
My basement is 7' high and has a dirt(red clay)floor. The beam spans 16' sitting a brick wall/foundation as well as one post. It's 7 5/8" tall and 2" wide. My beams are 24" o.c. The beam supports 2 load bearing walls(1st/2nd floor) inside the house..not outside wall. And BEST OF ALL..my gas boiler sits right under this beam so i have litte room to place any jacks or supports.
How do I support the weight of 2 plaster walls so i can replace this beam.
p.s. I work on cars and I can rebuild an engine in my sleep...This house stuff is still new to me so forgive me if i dont understand some of the terms you might use
~Dave

Just to clarify; Do the floor joist butt into the beam or sit on top of it? If they sit on top the beam do they join there or are they one pc. Your beam sits directly on the foundation wall with a small center post, small unless it is steel. What is the post sitting on?

In most cases, you may be better off to ad to the structure rather than trying to replace. Can you get me a picture of the area you are concerned with?

Very basic principle it to build a temporary supporting wall on one or both side of the beam. Then remove the beam. Install new one. Remove temporary walls.

Consider sistering in adjacent beems, support those and you can probably leave the existing beam in place.

Your always better off to get all the rot out and it is never the easy way!!

I personaly would not concider this a first time DIY project and would hire it out. I've done it many times because we did nothing but work on 100 plus year old homes and churches for many years. It's going to require several HD jacks that once you buy them you may never need again, you have to know where to place the beams and jacks, just how much to lift them at a time ect. ect. Done wrong and the walls above will crack and all the doorways will be out of wack.
You did get the place treated with boron to kill the powder post beetles, right? I also would suggest getting a termite treatment.
Wood does not just rot on it's own, did you find out what was causing it?
Without someone being there to look at this it's near imposable to give you the correct advice, every situation is differant.