Drain pineapple, reserving syrup.
Combine rhubarb, sugar, and water; cover and cook just till tender, about 5 minutes.
Drain thoroughly, reserving syrup.
Combine pineapple and rhubarb syrups; add water to make 3 1/2 cups.
Heat to boiling; add gelatin and stir to dissolve.
Add lemon juice; cool.
Chill till partially set.
Fold in pineapple, rhubarb, and nuts; pour into 6-cup ring mold.