A method safer than chemical solvants for 
removing tape "gunk" is tape itself, believe it 
or not. Get a fresh piece of tape and stick it to 
the sticky gunk on the cel. When you peel it back 
off, a good portion of the sticky gunk will come 
with it. Repeat until the cel is clean.

When you can, it's better to avoid chemicals. . .

Many Sharp Smiles,
--Drac