Supposing the reaction to be:

2 NH3 + 3 O2 + 2 CH4 → 2 HCN + 6 H2O

(5.11 × 10^6 g NH3) / (17.03056 g NH3/mol) × (2 mol HCN / 2 mol NH3) = 300049 mol HCN

(5.11 × 10^6 g O2) / (31.99886 g O2/mol) × (2 mol HCN / 3 mol O2) = 106462 mol HCN

(5.11 × 10^6 g CH4) / (16.0425 g CH4/mol) × (2 mol HCN / 2 mol CH4) = 318529 mol HCN

Of the three reactants, O2 would produce the least amount of product, so O2 is the limiting reactant.

(106462 mol HCN) × (27.0253 g HCN/mol) = 2877167 g = 2.88 × 10^3 kg HCN

(5.11 × 10^6 g O2) / (31.99886 g O2/mol) × (6 mol H2O / 3 mol O2) × (18.01532 g H2O/mol) =

5753848 g = 5.75 × 10^3 kg H2O