Follow up:
I got impatient and ripped into the wall. It is post and beam (timber-frame). Now I just have to figure out if the existing beam is enough to span 15'.

"...figure out if the existing beam is enough to span 15'."

Possible, but not likely. The beams are designed around the bent spacing and layout.
Removing posts in a post & beam building is usually a large problem.
While wood may appear massive, the loads they are carrying are also large.

A PE with experience in wood design should be able to take some quick measurments and come up with a decision.
Part of the problem is that changes must conform to the present building codes for load caculations. Even wood has changed markedly since 1885 and you need lab testing to use anything but the modern numbers for material charecteristics. Since you are unlikely to have any samples around to perform testing you get caught in a 'catch-22' situation.

There are no posts in the center of this wall only the posts on either side. I talked with a timber-frame house/barn builder and he recommended a book with load tables. I followed the directions and came up with a beam that needed to be 8" x 8" to carry the load (I took some statics classes and am engineer, just not civil). However, the beam's actual dimensions are a true 7.5" deep x 5" wide. So, I believe I'm going to need either a post in the center (according to my calculations the beam can span up to 10'-11') or another beam to assist the existing one.

You can also make a flitch plate beam.
A relatively thin steel section is sandwiched betwen two wood members.
The wood prevents buckling of the thin steel while the steel provides extra shear strength to the beam.
Through fastening schedues are very important in flitch beams.

You can use a demo version of Superbeam for free for thirty days if you want to plug in actual dimensions and loads and see what you need.
It does flitche beams, just not the fastener schedule.